In transmission systems, the merging of two separate signals having two differing data rates, where a high speed data signal is combined with a lower speed data signal in the form of a supervisory signal is well known. For example Fee et al, in U.S. Pat. No. 5,956,165 discloses a system and method and apparatus that allow for the updating and transmittal of a low data rate signal providing ancillary network data as a rider on a high data rate optical signal. A subcarrier modulated signal is processed and updated by using a drop/insert facility without having to alter the high data rate signal component of the data signal. Since the subcarrier signal can be detected by inexpensive equipment without having to detect and demultiplex the high data rate signal, the approach described offers one solution to the problem of network monitoring. In this system ancillary network management information can be communicated regardless of the quality or loss of the high data rate signal. In addition, by updating the subcarrier signal rider in a cumulative manner, a complete history of the fiber link the modulated optical signal travels on is provided to network management.
U.S. Pat. No. 6,549,311 in the name of Hakki et al. discloses a wave division multiplexing system in which low frequency intensity modulation of the optical channels is avoided in order to reduce or substantially prevent inter-channel cross-talk of low frequency telemetry signals by phase modulating the low frequency telemetry signal so as to recover the transmitted signal at the receiver. The system includes multiple transmitters and receivers. Each transmitter includes an optical source, such as a laser, a radio frequency generator having a low frequency telemetry signal impressed thereon, and an optical phase modulator. The phase modulated low frequency telemetry signal generated by each transmitter is combined using a multiplexer before being amplified and transmitted through a single optical fiber. A demultiplexer receives the transmitted signal and separates it into respective optical channels based on the particular frequency of each transmitter. Respective signals for each optical channel are transmitted to an associated receiver. Each receiver includes a splitter for tapping off a portion of power into a first portion and a second portion. A data recovery device reestablishes data information from the first portion of power, while an interferometer and detector are used to recover the low frequency telemetry information from the second portion of power. Although these systems appear to perform their intended function, there is a need for a communications system that is capable of simultaneously transmitting low data rate supervisory signals with high speed line data wherein the line data rate may vary.
The communication system in accordance with this invention is relatively inexpensive to build and is less complex than many of the systems proposed by others. The system in accordance with this invention relies on the line data rate for the data signals and in dependence of the data rate the modulation index is maintained or varied. The lower line data rates use a larger modulation index, and vice versa. Such a control means is simple and can be implemented conveniently and cost effectively in the transmitter without requiring receiving feedback from the receiver on the signal error rate. This is a significant advantage. Furthermore, this invention provides a simple scheme which allows a same transmitter to be used to impose supervisory data signals on data signals that may have a variety of different line rates. Such a transmitter can be advantageous in a system wherein different line data rates are utilized throughout a network, for providing a network of transmitters and receivers having different line data rates, and wherein supervisory signals are provided with the line data information for transmission along an optical fiber.
It is an object of the invention to provide a transmitter that allows data and supervisory information to be combined for optical transmission to a receiver at a different locale.
It is an object of the invention to provide a simple and inexpensive transmitter, which is capable of transmitting data and supervisory information and different transmission rates and wherein a modulation index is varied in dependence upon a varying incoming data rate.